The Skie's the limit as far as the horizon line goes. If you want to use the RSE, you'll have to change your camera angle or hide the horizon. There's a couple ways to do this. You could use physical objects such as clouds or terrains, or you could use the RSE in conjunction with the background function under scene effects. Of course, you could skip the RSE and use the backround entirely. I know you've mentioned Photoshop before, you could use it to generate a large gradient image and use that as a background.

These samples show before and after. I used the RSE in conjunction with the Gradient Color function as a background for the after images.